This is a multi-center, randomized, study to compare Iodine I 131 Tositumomab therapeutic regimen to Ibritumomab Tiuxetan therapeutic regimen in the treatment of patients with relapsed or transformed follicular non-Hodgkin's B-cell lymphoma. A total of 350 patients, approximately 175 patients per arm, will be enrolled at 30 to 40 sites in the United States.